Type 1 fimbriae-mediated adherence of Actinomyces viscosus T14V to saliva-treated hydroxyapatite (SHA) involves the recognition of proline rich proteins in the acquired pellicle whereas the type 2 fimbriae-mediated coaggregations of strain T14V or A. naeslundii WVU45 with Streptococcus oralis 34 depends on a Gal/GalNAc-reactive fimbrial lectin. The gene for the structural subunit of each different fimbria encoded a protein of 533 to 535 amino acid residues beginning with a cleavable signal sequence of 30 or 32 residues. Each mature subunit (M(r) equals about 54,000) was predominantly hydrophilic except for a carboxy terminal segment that was identified as a potential membrane spanning domain. The different Actinomyces spp. fimbrial subunits showed no significant global homologies with other proteins including the pilins of gram negative bacteria, but high homology was noted between the type 2 subunits of strains T14V and WVU45 (79% amino acid sequence similarity) and lower homology between either type 2 subunit and the type 1 subunit of strain T14V (49% amino acid sequence similarity). Receptors for the type 2 fimbrial lectin of Actinomyces spp. were detected on all strains of S. oralis (12/12) but less frequently or not at all on other viridans streptococci. Antigenically distinct linear polysaccharides, each composed of a different, phosphodiester linked repeating unit, were identified as the receptor structures on S. oralis strains 34, 10557 and C104 and S. mitis J22. As with strains 34 and J22, structural studies of the antigenically distinct receptor polysaccharides of strains 10557 and C104 have shown that their oligosaccharide repeating units have Gal(beta)1->3GalNAc and GalNAc(beta)1->3Gal, respectively, at the proposed site of lectin recognition. Each of these structures has been associated with the receptor activity of specific mammalian cell surface glycoproteins and glycoplipids, and the apparent mimicry between bacterial and host cell receptors provides a possible explanation for the failure of antibodies to be directed toward the receptor regions of these bacterial polysaccharides.